Electric power transmission
Electric-power transmission is the bulk transfer of electrical energy, from generating power plants to Electrical substation
Electrical substation
A substation is a part of an electrical generation, transmission, and distribution system. Substations transform voltage from high to low, or the reverse, or perform any of several other important functions...

s located near demand centers. This is distinct from the local wiring between high-voltage substations and customers, which is typically referred to as electric power distribution
Electric power distribution
File:Electricity grid simple- North America.svg|thumb|380px|right|Simplified diagram of AC electricity distribution from generation stations to consumers...

Transmission lines, when interconnected with each other, become transmission networks. In the US, these are typically referred to as "power grids" or just "the grid", while in the UK the network is known as the "national grid." North America has three major grids: The Western Interconnection
Western Interconnection
The Western Interconnection is one of the two major alternating current power grids in North America. The other major wide area synchronous grid is the Eastern Interconnection...

; The Eastern Interconnection
Eastern Interconnection
The Eastern Interconnection is one of the two major alternating current power grids in North America. The other major interconnection is the Western Interconnection...

 and the Electric Reliability Council of Texas (or ERCOT) grid.

Historically, transmission and distribution lines were owned by the same company, but since the turn of the 21st century many countries have liberalized
Electricity liberalization
Electricity liberalization refers to the liberalization of electricity markets. As electricity supply is a natural monopoly, this entails complex and costly systems of regulation to enforce a system of competition....

 the electricity market
Electricity market
In economic terms, electricity is a commodity capable of being bought, sold and traded. An electricity market is a system for effecting purchases, through bids to buy; sales, through offers to sell; and short-term trades, generally in the form of financial or obligation swaps. Bids and offers use...

 in ways that have led to the separation of the electricity transmission business from the distribution business.

Transmission lines mostly use high-voltage three-phase
Three-phase electric power
Three-phase electric power is a common method of alternating-current electric power generation, transmission, and distribution. It is a type of polyphase system and is the most common method used by grids worldwide to transfer power. It is also used to power large motors and other heavy loads...

 alternating current
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....

 (AC), although single phase
Single-phase electric power
In electrical engineering, single-phase electric power refers to the distribution of alternating current electric power using a system in which all the voltages of the supply vary in unison. Single-phase distribution is used when loads are mostly lighting and heating, with few large electric motors...

 AC is sometimes used in railway electrification system
Railway electrification system
A railway electrification system supplies electrical energy to railway locomotives and multiple units as well as trams so that they can operate without having an on-board prime mover. There are several different electrification systems in use throughout the world...